The present invention relates to a device for compensating for nonlinearity of amplifiers by use of a predistortion principle and a nonlinear signal generating circuit using such a device. More particularly, the present invention is concerned with a nonlinear compensating device of the type using a distortion generating circuit in which diodes are built in to serve as distortion generating elements.
Heretofore, a nonlinear signal generating circuit of the kind described has been implemented with a circulator. Specifically, a circulator is provided with a reflection port in addition to an input and an output port. Connected between the reflection port and ground is a parallel connection of two parallel diodes which are connected with polarities opposite to each other and a terminating resistor which is parallel to the diodes. The diodes serve as nonlinear elements. A radio frequency (RF) signal coming in through the input port is terminated by such a parallel circuit so that only nonlinear components generated by the diodes appear on the output port. A drawback with this kind of nonlinear signal generating circuit is that the bandwidth available is determined by that of the circulator and, therefore, cannot be wide enough as desired. Another drawback is that a magnet which is necessarily included in the circulator makes it difficult to implement the construction with a planar circuit and, thereby, obstructs the implementation of the circuit with a microwave integrated circuit. In addition, circulators having the same impedance are difficult to produce without adjustment and this, too, obstructs the implementation of the circuit with a microwave integration circuit.
On the other hand, a prior art nonlinear compensating device in which the above-described nonlinear signal generating circuit is installed to serve as a distortion generating circuit includes a power distributor, or power splitter, having an input terminal and a power combiner having an output terminal. An input signal is divided by the power distributor into two signals. One of the two signals is fed to the power combiner over a nonlinear route which includes the distortion generating circuit and a variable attenuator connected in series with the distortion generating circuit. The other signal is applied to the power combiner over a linear route which includes a delay line and a variable phase shifter connected in series with the delay line. The two signals delivered through the linear and nonlinear routes are combined by the power combiner in opposite phase and, then, fed out via the output port. Such a prior art nonlinear compensating device has some problems left unsolved. Specifically, since the linear and nonlinear routes lack symmetry, the phase difference between them has dependence upon frequency preventing the nonlinear compensating device from being provided with a wide bandwidth. In addition, since the individual structural elements are fabricated independently of each other and connected together by connectors and cables, difficulty is experienced in uniformizing the characteristics due to scattering in characteristic and others which are ascribable to the connection.